A 20,000 mAh bank holds about 74 Wh at 3.7 V; watt output depends on its USB/PD rating.
Shoppers ask this a lot because listings mix two ideas: stored energy and output power. Energy tells you how much charge the battery holds (watt-hours). Power tells you how fast it can move that energy (watts). One answers how long the bank can run a device; the other answers how fast it can charge a phone or laptop.
Energy Vs. Power: The Two Numbers You Need
Capacity in milliamp-hours is a measure of charge. To compare across devices you want energy in watt-hours. The quick rule: Wh = (mAh × V) ÷ 1000. For lithium-ion cells inside power banks the nominal cell voltage sits near 3.7 V. That puts a 20,000 mAh pack at about 74 Wh of stored energy. If the maker quotes capacity at 3.6 V you get 72 Wh. At 3.85 V you get 77 Wh. Multiply mAh by the internal cell voltage to convert charge into energy.
| Assumed Voltage | Energy (Wh) | What It Means |
|---|---|---|
| 3.6 V | 72 Wh | Many spec sheets round to 3.6 V. |
| 3.7 V | 74 Wh | Common nominal cell value. |
| 3.85 V | 77 Wh | High-voltage Li-ion chemistry. |
| 5 V | 100 Wh | Only valid if capacity is rated at 5 V output. |
Watts From A 20,000 mAh Power Bank – Real Numbers
The watt figure printed on the case tells you charge speed. A label like 18 W, 20 W, 30 W, 45 W, 65 W, or 100 W describes the highest power mode it can deliver to a single port under the right cable and protocol. That rating comes from voltage × current on the USB-A or USB-C port, not from the milliamp-hour capacity.
USB-C with Power Delivery supports stepped profiles. Older PD tops out at 100 W. PD 3.1 extends that ceiling to 240 W with higher voltage ranges. The bank you buy picks a subset of those modes, so check the fine print for its maximum per-port and total output.
Formula You Can Trust
Use this every time: watt-hours = milliamp-hours × volts ÷ 1000. To estimate run time, divide energy by device power. A 10 W device on a 74 Wh bank looks like 74 ÷ 10 ≈ 7.4 hours before conversion losses. Real results land lower, so plan a 10–15% margin.
Why The Same Battery Gives Different Results
Three variables shape real-world results: voltage conversion, cable/protocol, and the device’s draw curve. The cells store energy near 3.7 V. Your phone or laptop might be sipping at 5 V, 9 V, 12 V, 15 V, 20 V, or the new 28/36/48 V PD 3.1 modes. Converters inside the bank step the voltage up, which wastes a bit of energy as heat. Then the device controls how fast it can accept power based on its own thermal limits and battery state.
Voltage Matters More Than You Think
Marketing often lists milliamp-hours at the cell side. If a package claims “20,000 mAh,” that number is usually at ~3.7 V inside the pack. When you see claims like “20,000 mAh at 5 V,” that’s a different yardstick and will show a larger watt-hour number because the voltage is higher. Always pair mAh with a voltage when comparing two products.
Losses You Should Budget For
Charge and discharge aren’t perfect. Lithium-ion is efficient, yet DC-DC conversion and cable drops take a bite. If you treat a 74 Wh pack as 60–65 Wh available at the USB port under mixed use, expectations will line up with real life.
Reading Specs Without Guesswork
Check three lines on the box or spec sheet: energy in Wh, maximum single-port output in W, and protocol support (USB-A BC 1.2, USB-C PD/PPS or vendor modes). Together these tell you what the pack can store and how fast it can move that energy into your device.
Decoding USB And PD Labels
USB-A fast charge ports often top out near 12 W under the Battery Charging 1.2 rule set. USB-C with Power Delivery covers the stepped voltages from 5 V upward. PD 3.1 adds higher ranges that reach 240 W on capable chargers and cables.
For a clear overview of the standard, see USB Power Delivery 3.1. It explains the jump from 100 W to 240 W and why cable ratings matter.
How Long Will It Run My Stuff?
Use the same flow every time. First, convert to Wh. Next, shave a bit for losses. Then, divide by your device’s draw in watts. Loads aren’t flat; phones and many laptops pull less power near full.
| Label (W) | Voltage × Current | Good For |
|---|---|---|
| 5–12 W | 5 V × 1–2.4 A | Legacy USB-A, small devices, earbuds cases. |
| 18–20 W | 9 V × 2 A or 5 V × 3 A | Most phones at near-peak speed. |
| 27–30 W | 9–15 V × 2–3 A | Tablets, handheld consoles. |
| 45 W | 15 V × 3 A | Light laptops, Chromebooks. |
| 60–65 W | 20 V × 3 A | Many thin-and-light laptops. |
| 100 W | 20 V × 5 A | Pro laptops that accept 100 W USB-C. |
Travel Check: Airline Limits And Labels
If you fly, watt-hours matter because airlines check that number. In many regions packs up to 100 Wh go in carry-on; 101–160 Wh can need airline permission. That puts a 20,000 mAh class pack under the common 100 Wh line in normal cases. If your bank lists only milliamp-hours, use the conversion to find its Wh label before you pack.
For official wording see the FAA lithium battery rules. Airport staff go by that watt-hour label, not just milliamp-hours.
Buying Tips For Real-World Results
Match Output To Your Device
Pick a pack whose single-port watt rating meets or beats your device’s intake. A phone is happy at 18–30 W. A handheld console likes 27–45 W. Many laptops smile at 60–100 W. If the bank’s top mode is lower than the device wants, it still charges, just more slowly.
Check The Cable Rating
Fast modes need the right cable. A 100 W charge path needs a 5 A USB-C cable. The new 140–240 W modes need e-marked 5 A cables certified for those levels. If the cable can’t carry the current, the charger and device fall back to a lower step.
Look For The Wh Number On The Label
Good makers print watt-hours on the case. That single number bypasses mAh confusion and helps at security. When two packs list the same mAh, the one with the higher Wh is storing more energy because it is measured at a higher internal voltage.
Worked Walkthrough: From mAh To Run Time
Step 1: Convert Capacity To Energy
Take 20,000 mAh at 3.7 V. Multiply and divide: 20,000 × 3.7 ÷ 1000 = 74 Wh.
Step 2: Budget For Losses
Set aside 15%. Usable energy ≈ 74 × 0.85 = 63 Wh.
Step 3: Divide By Device Power
If your device averages 12 W, then 60 ÷ 12 ≈ 5 hours. If it averages 30 W, plan for about 2 hours total.
No-Nonsense Clarifications
Does Higher mAh Mean Faster Charging?
No. mAh speaks to energy stored. Charge speed comes from output watt ratings and supported protocols.
Why Do Some Sellers Quote 100 Wh For This Class?
They may be rating capacity at 5 V on the output side. Many brands rate at cell voltage. Both can be true; they use different reference points. Compare Wh only when the voltage basis is clear.
What If I Need Laptop-Level Power?
Pick a pack that lists 60 W, 65 W, 100 W, or more on USB-C. Also match the cable and confirm your laptop accepts that mode over USB-C.